The present invention relates to a signal detecting circuit.
A magnetic detecting circuit for detecting a magnetic field in a predetermined position by a Hall element using a Hall effect is used as a position sensor such as an open/close detecting device installed in a foldable mobile phone. The magnetic detecting circuit includes a Hall element which outputs a voltage in proportion to a magnetic field or a magnetic flux density; an amplifier which amplifies an output voltage from the Hall element; and a comparator which determines the presence or absence of an output from the Hall element based on an output from the amplifier.
The magnetic detecting circuit can be made of silicon to be formed into a one-chip Hall IC, which is favorable for device miniaturization. Here, a silicon Hall element outputs a very small voltage, which causes a problem with noise in the detecting circuit or an offset voltage of the amplifier or the comparator.
As a magnetic field sensor for reducing the influence of the input offset voltage in the amplifier, there has been proposed a magnetic field sensor which includes: a Hall element; a switching circuit for outputting the output voltage from the Hall element by switching so as to reverse the polarity at a first timing and at a second timing; an amplifier for outputting by amplifying an output from the switching circuit; a capacitor, one end of which is connected to an output terminal of the amplifier and holds an output voltage from the amplifier; and a switch which is connected to between the other end of the capacitor and the other output terminal of the amplifier (for example, see Japanese Patent No. 3315397).
Assuming that an amplification factor of the amplifier is β, and an input offset voltage is Voff, an output voltage Vh from the Hall element is inputted into the amplifier at a first timing, and a voltage V1=β (Vh+Voff) is outputted. When the switch is closed, the voltage V1 is stored in the capacitor.
Then, at a second timing, an output voltage −Vh′ having a reverse polarity to the one at the first timing is inputted from the Hall element into the amplifier. When the switch is opened, a voltage V2=β (−Vh′+Voff) is outputted from the amplifier. Here, the output V from the magnetic sensor is the sum of the voltage V2 and the voltage −V1 of amplifier-side terminal of the capacitor.
As a result, V=V2+(−V1)=−β (Vh+Vh′) is obtained, which indicates that the influence of the input offset voltage Voff in the amplifier is cancelled.
In addition, the output voltages Vh and Vh′ from the Hall element contain an in-phase effective signal component and an anti-phase element offset voltage. Thus, by adding Vh and Vh′, it is possible to cancel the influence due to the element offset voltage of the Hall element as well.
However, there is a problem in that the magnetic detecting circuit using this magnetic sensor does not have a function to cancel the offset voltage of the comparator, thereby producing an inconsistent comparison value.